Fabrication of electronic circuit elements using liquid deposition techniques is of profound interest as such techniques provide potentially low-cost alternatives to conventional mainstream amorphous silicon technologies for electronic applications such as thin film transistors (TFTs), light-emitting diodes (LEDs), RFID tags, photovoltaics, and the like. However the deposition and/or patterning of functional electrodes, pixel pads, and conductive traces, lines and tracks, which meet the conductivity, processing, and cost requirements for practical applications have been a great challenge.
Solution-processable conductors are of great interest for use in such electronic applications. Silver nanoparticle-based inks represent a promising class of materials for printed electronics. For example, silver nanoparticles have unique optical, electrical, and thermal properties and can be incorporated into products that range from photovoltaics to biological and chemical sensors. Furthermore, conductive inks, pastes and fillers can utilize silver nanoparticles for their high electrical conductivity, stability, and low sintering temperatures.
However, most silver (and gold) nanoparticles often require large molecular weight stabilizers to ensure proper solubility and stability in solution. These large molecular weight stabilizers inevitably raise the annealing temperatures of the silver nanoparticles above 200° C. in order to burn off the stabilizers, which temperatures are incompatible with most low-cost plastic substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) that the solution may be coated onto and can cause damage thereto.
Furthermore, current silver nanoparticle compositions may have adhesion issues with some substrates. Simply rubbing and/or contacting the surface of the printed silver features may thus inevitably damage the printed features from such silver nanoparticle compositions. Thus, the poor adhesion of the silver nanoparticle composition may limit its use in certain applications, such as, for example, printed antennas.
Additionally, nanoparticles can require special handling during processing. For example, in a conventional method 100 for preparing nanoparticles-based inks as shown in FIG. 1, dried silver nanoparticles, such as nanoparticles stored in powder form are utilized. The nanoparticles can be formed by a method 101. According to method 101, a stabilizer can be dissolved in a first solvent 103, a reducing agent can be added the first solvent 105, a metal salt can be added to the first solvent 107, and the stabilized metal nanoparticles can form, such as by precipitation, in the first solvent 109 to form a slurry of metal nanoparticles. The metal nanoparticles can then be formed into a wet cake 110, such as by filtering the slurry then actively dried 112, such as by vacuum drying, air drying or the like. The dried silver nanoparticle powder can be weighed out in a fume-hood. Meanwhile, a polyvinyl alcohol derivative can be added 111 with a second solvent. The dry nanoparticles can then be added 121 with the second solvent. For example, the powder can be added to decalin and mixed for a period of time to be fully dispersed. Appropriate mixtures are determined by the quality of dispersion therein as verified by predetermined surface roughness and conductivities, as well as, predetermined silver content in the dispersed phase by ash technique.
Occupational exposure limits are well known for larger particles of similar chemical composition but exposure limits for airborne exposure to engineered silver nanoparticles have not been readily established. Published work in nanotoxicology describes studies related to high concentration of silver nanoparticles in rats or different microorganisms which could be difficult to translate to a realistic human chronic exposure scenario. The human risk associated with workplace air concentrations of silver nanoparticles and their release mechanisms and concentrations is still unknown.
In addition to the lack of identified exposure limits for materials such as silver nanoparticles, for example, those having average particle size of <10 nm, properly designed personal protection equipment (masks, etc.) to prevent exposure is not readily available. Accordingly, handling such small particles, especially during the manufacture of large quantities of inks using dried nanoparticles is challenging and must be presumed to be unsafe.
There is, therefore, a need for a safer method of manufacturing inks that overcomes the challenges of current manufacturing processes that utilize dried nanoparticles.